It has been known to form poly(arylene sulfides) represented by poly(p-phenylene sulfide) into biaxially-stretched films. Incidentally, the friction coefficient, namely, slip properties between plastic films themselves or between a plastic film and another material generally give significant influence to the efficiency of work when the plastic films or film is processed. When surface coating, printing or wrapping is performed using a film having poor slip properties, a large tensile force is applied to the film so that difficulties are encountered in feeding the film or taking it up into a roll. When the film is employed as a capacitor film in particular, the film is processed through many take-up and rewinding steps in the course of its assembly into a capacitor element. If the slip properties of the film are poor, the film may be torn, stretched and whitened, and/or caused to develop wrinkles. Due to these problems, the film cannot be assembled successfully into a capacitor element or even if it looks as if assembled into a capacitor element, the capacitor element involves one or more fatal defects and cannot be used practically as a capacitor element. On the other hand, unduly good slip properties may also deteriorate the efficiency of work in some instances, including the occurrence of irregular winding by wa of example. It is therefore necessary to improve slip properties of a film, in other words, to render the film slidable easily to a certain extent. These easily slidable properties will hereinafter called "high lubricity" for the sake of brevity. For poly(p-phenylene sulfide) films, it is also essential to impart high lubricity from the practical viewpoint. It has heretofore been known to control the surface roughness of a film by adding, as a filler, one or more inert inorganic materials in the form of fine powder, for example, calcium carbonate, silica, alumina, carbon black, glass, calcium phosphate and/or the like (Japanese Patent Publication No. 5101/1984). The addition of such an inorganic filler however results in a reduction in the dielectric strength of a poly(p-phenylene sulfide) film, thereby making the film somewhat insufficient for use as a capacitor film or electrical insulating film in view of the standard of properties required presently for such an application.
The present inventors have already proposed to add poly-4-methylpentene-l in place of a conventional inorganic filler with a view toward imparting high lublicity to a poly(arylene sulfide) (Japanese Patent Application No. 75827/1986). The resultant composition has however been found not to be fully satisfactory in processability such as extrusion stability.
On the other hand, polystyrene films have excellent electrical properties and are used as insulating films, especially, as capacitor films. They however have a drawback that their heat resistance is inferior.
It has also known to blend a polystyrene-based resin with a poly(arylene sulfide) in order to modify the latter resin, as disclosed in Japanese Patent Publication No. 13469/1978 by way of example. However, the poly(arylene sulfide) employed in the above patent publication is such that could be obtained in accordance with a process disclosed, for example, in Japanese Patent Publication No. 3368/1970, namely, by reacting a polyhalogenated aromatic compound and an alkali metal sulfide at an elevated temperature in a polar organic medium. The poly(arylene sulfide) obtained by such a process has an extremely low polymerization degree and a low melt viscosity, so that it cannot be formed into biaxially-stretched films, to say nothing of its extrusion into films. In addition, the poly(phenylene sulfide) of the low polymerization degree still cannot provide useful films even when it is crosslinked or cured in order to permits its injection. The resin composition described in Japanese Patent Publication No. 13469/1978 has been provided with a view toward improving the moldability and impact resistance of a poly(arylene sulfide) without lowering its tensile property. The resin composition is said to be moldable into articles for various application fields. Its formation into films is however not disclosed there.
It has also been known to blend one or more of various synthetic resins with a poly(arylene sulfide) having a high molecular weight in general. It has however been unknown to form a resin composition, which is composed of a poly(arylene sulfide) and a polystyrene-based resin, into a biaxially-stretched film.
As has been described above, the reduction of dielectric strength characteristics has been unavoidable so long as high lubricity is imparted to a poly(arylene sulfide) film by the addition of an inorganic filler. Furthermore, it has not been known to form a resin composition, which is composed of a poly(arylene sulfide) and a polystyrene-based resin, into a biaxially-stretched film.